This invention relates generally to fiber-optic gyroscopes and more specifically to the pseudorandom bit sequence generators that are used in such apparatus.
Fiber-optic gyros measure rate of rotation by determining the phase difference in light waves that propagate in opposite directions through a coil wound with optical fiber. Light waves that propagate through the coil in the direction of rotation take a longer time than light waves that propagate through the coil in the direction opposite to the direction of rotation. This difference in time, measured as the phase difference between counter-propagating light waves, is proportional to the angular velocity of the coil.
A typical block diagram for a fiber-optic gyro is shown in FIG. 1. A light source 2 supplies a reasonably coherent light beam to the optical-fiber interferometer 4 which causes the input light beam to be split into two light beams that are fed into opposite ends of an optical fiber 51 configured as a coil 56. The light beams emerging from opposite ends of the optical fiber are recombined into a single output light beam, which feeds into the detector 6.
In more detail, the light beam W.sub.i from light source 2 passes into port A and out of port C of directional coupler 52 and then into port A and out of ports C and D of directional coupler 54. Thus, two counter-propagating light beams W.sub.1 and W.sub.2 are established in coil 56. The counter-propagating light beams W.sub.1 and W.sub.2 are phase-modulated by modulator 58 and then pass into ports C and D of directional coupler 54 where they are combined into a single light beam W.sub.o that exits through port A. The combined light beam W.sub.o passes into port C of directional coupler 52, out of port B, and then into detector 6.
The output of the detector 6 is given by ##EQU1## where I.sub.o is the peak light intensity and .theta.(t) is the phase difference between the two beams expressed as a function of time.
The phase difference .theta.(t) typically takes the form EQU .theta.(t)=[.PHI.(t)].sub.mod 2.pi. -[.PHI.(t-.tau.)].sub.mod 2.pi. +.phi..sub.S +2.pi.n (2)
where .PHI.(t) is the phase-modulation generating function and .PHI.(t).sub.mod 2.pi. is the phase modulation introduced by a phase modulator at one end of the fiber-optic coil in the interferometer 4, .tau. is the propagation time through the fiber optic coil, and (.phi..sub.S +2.pi.n) is the so-called Sagnac phase resulting from the rotation of the fiber-optic coil about its axis. The integer n (called the Sagnac fringe number) is either positive or negative and the Sagnac residual phase .phi..sub.S is constrained to the range -.pi..ltoreq..phi..sub.S &lt;.pi..
The output of the detector 6 is converted to digital form by the analog-to-digital converter 8 and then processed in the digital processor 10 to yield at the output a measure of the rate and angle of rotation of the interferometer 4. In addition, the digital processor 10 generates a phase-modulation generating function .PHI.(t), the modulo-2.pi. portion of which is converted to analog form by the digital-to-analog converter 12 and supplied to the phase modulator in the interferometer 4.
The phase-modulation generating function .PHI.(t) typically consists of a number of phase-modulation components among which are .PHI..sub.SE (t) and .PHI..sub.M (t). The phase-modulation component .PHI..sub.SE (t) is typically a stepped waveform with steps that change in height by -.phi..sub.SE at .tau. intervals where .phi..sub.SE is an estimate of .phi..sub.S. Thus, the .PHI..sub.SE (t) modulation cancels in large part .phi..sub.S. The accurate measurement of the uncancelled portion of the Sagnac residual phase .phi..sub.S is of great importance in that it is the quantity that is used in refining the estimate of the Sagnac phase and generating the .PHI..sub.SE (t) phase-modulation component.
The accurate measurement of the uncancelled portion of the Sagnac residual phase is greatly facilitated by choosing the .PHI..sub.M (t) phase-modulation component such that [.PHI..sub.M (t)-.PHI..sub.M (t-.tau.)] is equal to j.phi..sub.M where the permitted values of j are the values -1 and 1 and .phi..sub.M is a predetermined positive phase angle somewhere in the vicinity .pi./2 radians where the slope of the cosine function is greatest. This effect can be achieved, for example, by having .PHI..sub.M (t) be a square wave with amplitude .pi./2 and period 2.tau..
A method of fiber-optic gryo modulation using a pseudorandom bit sequence to control the sign j of the phase modulation quantity j.phi..sub.M was disclosed by Spahlinger in U.S. Pat. No. 5,123,741. This pseudorandom modulation approach improved gyro bias by canceling out cross-coupled electronic errors, which rectify in conventional deterministic modulation schemes. This approach was carried a step further by Mark & Tazartes in an invention disclosed in U.S. Pat. No. 5,682,241 wherein a method of random overmodulation (i.e. .phi..sub.M greater than .pi./2) simultaneously reduced bias errors (via pseudorandom modulation) and random walk (via overmodulation--see U.S. Pat. No. 5,530,545).
In the early development of pseudorandom modulation it was determined that sequences with short repetition periods were undesirable because of their relatively poor autocorrelation properties. Sequences with sufficient length (typically spanning on the order of 1 second) are used for fiber-optic gyro modulation. However, due to the random walk properties of pseudorandom sequences, it was found that these sequences generated large low-frequency components. These low-frequency components are undesirable because they do not transmit well through AC-coupled circuits, as is usually the case with the optical detector circuitry used in fiber-optic gyros. Further, the low frequencies imply that for extended periods of time the gyro may operate predominantly with one modulation sign. During these periods, the gyro signal may be subject to offset drifts, intensity drifts, or gain drifts.